At small businesses, homes (houses, condos, and apartments), hotel rooms, hospital rooms, college dorms, offices, etc. often visitors arrive at the door, but the door is not answered because the occupant is not in or is otherwise unable or unwilling to answer the door.
Sometimes the visitor will leave a note attached to the door to say that they were there and to the describe the nature of the visit. To leave a note, the visitor must have paper, pen, and tape to attach the note to the door. In addition, wind, vandals or rain can detach the note so that it is lost.
Also, the attachment of a note to a door may signify that the home or office is unoccupied to those seeking an opportunity to burglarize it. The problem of answering a phone when the occupant is out has been solved by the telephone answering machine. Although elaborate intercom systems for manually answering door inquiries are well known, simple voice message recorders for use at the door are not available.